The proliferation of mobile wireless devices in recent years has lead to an interest in reducing the power consumption of the circuits used within both the mobile phone and wireless devices generally. In particular, the realisation of a CMOS RF down-converter would allow an increase in transceiver integration to be achieved. Typically, a limitation of the performance of a mobile wireless device is the period for which the battery can reliably supply power to the device between re-charges or battery replacements. Suitably, the design of mixers that have a low power consumption would increase the working period of the mobile wireless device. Alternatively, by producing circuits that consume less power, the capacity, and consequently the size, of the battery can be either reduced for a given operational life-time, which also leads to a weight reduction, or maintained for an increased operational life-time for the same weight.
It is well understood within the art that the design of mixers requires a compromise to be reached between, for example, conversion gain, local oscillator power, linearity, noise figures, port-to-port isolation, voltage supply and current consumption. A double balanced Gilbert mixer, as is well known within the art, is typically used to meet the varied requirements imposed upon an RF mixer for mobile, that is, battery powered, devices.
However, a typical Gilbert cell with differential inputs comprises a stack of at least three levels of transistors between Vdd and ground. Due to the stacked transistors commonly found within a Gilbert cell, these cells usually require more than at least one volt to operate correctly and they have a low gain.
It is an object of the embodiments of the present invention at least to mitigate some of the problems of the prior art.
An object of embodiments of the present invention provides a mixer that is capable of being operated via a single battery cell having a nominal voltage of 1.2 volts. A further object of embodiments of the present invention provides a mixer that has very low power consumption for use in, for example, direct conversion radio receivers.
A still further object of embodiments of the present invention is to provide a mixer that has a relatively high gain and that can be fully integrated onto a silicon chip.
Another object of embodiments of the present invention is to provide a mixer having an acceptable degree of port-to-port isolation, that is, to have an acceptable degree of RF and local oscillator isolation.